Because people try messing with them and/or their territory... They are only doing what evolution has 'programmed' them to do, defend themselves so they can live on and reproduce. It's not like it's their life's goal to kill people.

It may not respond to tone of voice and body language since it is a reptile, but every animal in existence understands if pain comes along with a certain action not to do it again (except lemmings. Those have got to be the dumbest ************* ever)

What animals are you talking about? I'm assuming a wolf pack, little do people know an Alpha male of a wild wolf pack will not "punish" the other wolves, the only time this behavior is seen is those kept in captivity by zoo's or small wildlife preserves, only than will they attack other wolves, and this is only because they become super aggravated and territorial in small enclosures.

In a wild wolf-pack, the only time an Alpha male shows his dominance is when challenged by another wolf out of the pack, or if one challenges him within the pack.